Work is being carried on along three lines: (1) Observations are being made on the rates of uterine blood flow as estimated by the diffusion-equilibrium technique and the maternal blood levels of estrone, estradiol and of progesterone in unstressed ewes at known stages in gestation. (2) Observations are also being made of the change in the rate of uterine blood flow (ml/min) estimated by the Fick principle, and in the blood levels of estrone, estradiol and progesterone in near term ewes after killing the fetus in utero by air injection. (3) A comparison is being made of the rate (ml/kg/min) of the oxygen consumption of the pregnant uterus of the ewe and the growth rate of the fetus or fetuses. The oxygen consumption is estimated as the product of the a-v oxygen difference across the uterus and the blood flow. The growth rate of the fetus is estimated from the increase in its circulating plasma volume.